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Serving: IA
Managing Weed Resistance
reading a herbicide label
HOW TO READ A LABEL: Herbicide group numbers are shown on the front page of a herbicide label. This particular herbicide product has three different sites of action, so three different group numbers are listed on its label.

Plan an effective herbicide program

Stopping, and preventing, weed resistance is possible, provided you follow a few basic tactics.

By Meaghan Anderson

Have you planned an effective weed control program this year? Weeds are developing herbicide resistances faster than ever. No new herbicides are expected to solve these resistance issues in the future, so it’s important to use current options as effectively as possible.

Careful herbicide program planning and use of non-herbicide management tactics when possible can help keep herbicides working. Careful herbicide program planning includes using multiple herbicide sites of action that are effective against problem weeds.

Use multiple sites of action

The first step to battling herbicide resistance is planning a program with multiple herbicide sites of action. The herbicide site of action is the specific molecule that the herbicide binds to; this binding disrupts a biological process (the mode of action) and results in death or injury to the plant. Important herbicide sites of action and their corresponding herbicide group numbers are listed in the table below.

Herbicide groups are a relatively new way of determining the site of action of the many herbicides on the market. Each site of action has been assigned a number. Most labels prominently display this group number on the product label. If a product includes more than one site of action, the label will have multiple group numbers listed, as the example above shows.

Keeping track of the herbicide group numbers is the simplest way for farmers to keep track of the different sites of action they’re using in their herbicide program.

The “2016 Herbicide Guide for Iowa Corn and Soybean Production” from Iowa State University contains herbicide group numbers for products with only one active ingredient (Page 18) or group numbers for premixes with multiple herbicide group numbers (Pages 19-20). This document can be found at

Effective against target weeds

Using multiple herbicide groups isn’t enough; the herbicides you use must be effective against target weeds. These target weeds may be the same across all fields or may be different depending on the field or area of the field. When considering herbicide group effectiveness, it’s important to determine whether the herbicide is labeled to control the target weed and whether your farm or field already has weeds resistant to that specific herbicide group.

Farmers can consult herbicide labels to check whether a herbicide is labeled to control their problem weed. For example, HG 1 contains only grass-killing herbicides, making them ineffective against broadleaves. HG 13 contains clomazone, a product labeled for preemergence sprays in soybeans. It would provide unacceptable waterhemp control if applied alone. HG 22 contains paraquat. It is a very effective nonselective contact herbicide but cannot be sprayed on emerged waterhemp in a growing corn or soybean field.

Weeds known to be resistant in Iowa are listed at The presence of resistant weeds will vary from field to field, often based on individual management tactics used by farmers. Iowa has biotypes of waterhemp resistant to HG 2, HG 5, HG 9, HG 14 and HG 27. Other resistant weed species in Iowa include giant ragweed, horseweed and giant foxtail.

Waterhemp is Iowa’s weed most efficient at accumulating multiple resistances. In Iowa, at least one population has been identified that is resistant to groups 2, 5, 9, 14 and 27 herbicides. Most waterhemp in Iowa are resistant to HG 2, making those products ineffective against any waterhemp populations. It’s important to determine what herbicide groups you have relied on for managing waterhemp on your farm, and then carefully evaluate if they are still controlling weeds as effectively as in the past.

Passing rate test

Herbicide application rate selection is important in determining effectiveness as well. Many herbicide products and programs combine multiple herbicides and herbicide groups to provide broad-spectrum weed control and ease of use.

Sometimes premixes, even when used at full label rates, will provide lower rates of individual products compared to the stand-alone product. Evaluating herbicide programs to determine that all products are used at effective rates is a critical component in managing herbicide resistance to weeds. Reduced application rates of preemergence herbicides can be effective at preventing early-season competition by controlling the first flush of weeds, but the reduced rates greatly reduce the length of weed control provided.

The shorter residual control results in a larger weed population exposed to the postemergence herbicide program and increases the risk for weeds to develop resistance to the post-applied products. The shorter residual also reduces the window for timely application of the postemergence product.

In order to evaluate the rates of herbicides, compare rates of active ingredients provided by premix products to the rate of active ingredient provided by the stand-alone products at the use rate. The application rates and the concentration of active ingredients of both the premix and the standalone product are necessary to make these comparisons. This will require comparing the two herbicide labels and doing some math to find the active ingredient applied to each acre instead of the product application rate per acre.

Ideally, the rates applied in premix products should be the same as the single active product. However, manufacturers frequently reduce the rate assuming there will be additive action among the active ingredients, or to reduce costs. How much of a reduction in the application rate is acceptable is subjective, but a rule of thumb might be 25% to 30% reduction.

By doing the math, you will be more aware of the expected period of weed control with a preemergence herbicide program and be able to make better management decisions based upon that knowledge. You may also have the ability to spike herbicide applications with additional active ingredient to reach that 100% full rate, if the label allows.

Plan for the long term

To prolong herbicide efficacy, it’s important to fit cultural and mechanical management tactics in wherever possible to supplement herbicides. Weed management will likely continue to rely heavily on herbicides, but serious consideration of alternative tactics is important to preserve the efficacy of the herbicides for as long as possible. This includes developing programs that include multiple, effective herbicide groups at appropriate rates.

It’s important to consider a multiyear plan to avoid falling into the habit of using the same herbicide program repeatedly. It is equally important to consider what alternative tactics fit into your specific weed management system: tillage, narrow-row soybeans, increased seeding rates, hand weeding, or other techniques. These will become even more important in the future as waterhemp and other weeds continue to stack up resistances.

- Anderson is the ISU Extension field agronomist for east-central Iowa. Contact her at

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